Packet forwarding devices, such as layer 3 or IP routers, typically use one or more tables to store packet forwarding information. For example, layer 3 packet forwarding devices may include a hardware-implemented layer 3 host table that typically stores entries corresponding to hosts that are directly connected to the packet forwarding device. Each entry may include an IP address of a locally connected host and corresponding forwarding information for the locally connected host. When a packet arrives at the packet forwarding device, a lookup may be performed in the layer 3 host table based on the destination IP address in the packet. If a matching entry is located, the corresponding packet forwarding information is extracted from the layer 3 host table and used to forward the packet to the destination host. If an entry is not located, a miss occurs, and a slower lookup is performed using software.
Another type of table that may be maintained in a layer 3 packet forwarding device is a longest prefix matching (LPM) table. Each entry in a longest prefix matching table may include a subnet address and a subnet mask to be applied to destination IP addresses of incoming packets. When a packet is received, the subnet mask for each entry in the LPM table is applied to the destination IP address of the packet, and the masked address searched in the table. The entry with the longest prefix that matches the masked address in the received packet is used to extract next hop address information for the packet. Using an LPM table allows remote hosts to be identified by next hops without requiring entries in the LPM table for each individual remote host.
One problem with conventional packet forwarding devices is that space for the LPM and host tables is limited. For example, both tables are typically implemented in hardware to increase the number of hosts covered by hardware forwarding and therefore packet forwarding lookup speed. LPM tables are typically populated based on participation in routing protocols, such as routing information protocol (RIP), border gateway protocol (BGP) or open shortest path first (OSPF). When an LPM table becomes full, entries for remote hosts can be placed in the host table. However, the host table can also become full. When this occurs, packets received for destinations that are not present in the LPM or host tables must be forwarded using software, which results in longer lookup times.
In order to increase the routing efficiency of a layer 3 packet forwarding device, older entries in LPM and host tables may be replaced with newer entries. Conventionally, the replacement criteria for an entry include a least recently used (LRU) algorithm. That is, a host table entry that was used least recently may be replaced with a new entry for which a packet forwarding lookup fails. One problem with using an LRU algorithm to replace host table entries is that it is based on individual packets, which may result in excessive replacement of entries in the host table. For example, each time a miss occurs, an entry in the host table may be replaced. Such excessive replacement is undesired because replacing entries in the host table may be a processor intensive operation that requires a software copy of the host table be copied to hardware.
Another problem associated with using an LRU algorithm to replace entries in the host table is that such an algorithm does not take into account the volume of packets addressed to a destination. For example, an entry that was used 1000 times during a time interval more than one minute ago may be replaced by an entry that was used only once in the last minute. The high-volume entry may be more likely to be utilized in the future with a high volume than the low-volume entry. Thus, if the high-volume entry is replaced by the low-volume entry, multiple misses may occur before the high-volume entry can be re-added to the host table.
Accordingly, in light of these difficulties associated with conventional layer 3 packet forwarding devices, there exists a need for methods, systems, and computer program products for controlling updating of a layer 3 host table based on packet forwarding miss counts.